Allocation period determination for packet data

ABSTRACT

An allocation period for packet data in a packet data communication system is determined. Measurements from mobile stations are collected and a maximum allocation period for at least one possible base station for transmitting data packets to the mobile station is determined on the basis of the collected measurements. A base station and a corresponding allocation period are selected on the basis of the at least one determined maximum allocation period.

FIELD OF THE INVENTION

[0001] The present invention relates to the field of Radio ResourceManagement (RRM) for Code Division Multiple Access (CDMA) with specialfocus on packet services.

BACKGROUND OF THE INVENTION

[0002] In current packet data communication systems Soft HandOver (SHO)is used for downlink packet services, and packet scheduling is doneperiodically. However, studies have shown that SHO is not alwaysbeneficial for packet services.

[0003] From the interference point of view it is not desirable to haveSHO in the downlink. For instance, it is very rare that the averagepropagation loss coming from multiple Base Stations (BSs) are the same,resulting into poor multipath combining at the RAKE receiver.

[0004] Moreover, packet transmissions are shorter compared tocircuit-switched calls, and seamless connection is also not mandatory.Oftentimes, a packet Mobile Station (MS) is in the DTX (DiscontinuousTransmission) mode, and performing unnecessary SHO updates during DTXdoes not contribute to the system performance.

[0005] Hence, using SHO for downlink packet services does notnecessarily improve the system capacity but rather introduces a largeamount of signaling.

SUMMARY OF THE INVENTION

[0006] It is therefore an object of the present invention to improvepacket scheduling for a packet Mobile Station.

[0007] According to one aspect of the present invention, this object isachieved by a method of determining an allocation period for packet datain a communication system. The method comprises the step of collectingmeasurements from mobile stations. A maximum allocation period for atleast one possible base station for transmitting data packets to themobile station is determined on the basis of the collected measurements,and a base station out of the possible base stations is determined onthe basis of the at least one determined maximum allocation period.Then, the determined base station and a corresponding allocation periodare selected.

[0008] According to another aspect of the present invention, theabove-mentioned object is achieved by a control device for determiningan allocation period for packet data in a communication system. Thecontrol device comprises a control entity which is adapted to collectmeasurements from mobile stations and to determine, on the basis of thecollected measurements, a maximum allocation period for at least onepossible base station for transmitting data packets to the mobilestation. The control entity determines a base station out of thepossible base stations on the basis of the at least one determinedmaximum allocation period. Moreover, the control entity comprises ascheduling entity which selects the determined base station and acorresponding allocation period.

[0009] Further features of the present invention are defined in thedependent claims.

[0010] According to the present invention, Hard HandOver (HHO) withadaptive allocation periods is used. Accordingly, there is no more SHOin the downlink. This is a clear advantage since SHO decreases thesystem complexity and even capacity. According to simulations conductedby the inventors, downlink SHO requires more BS power. A remarkabledecrease of signaling inside the Radio Access Network (RAN) and in theair-interface is achieved.

[0011] According to the present invention, the packet allocation periodcan be adaptively determined, and may be made shorter near cellboarders.

[0012] In the following the present invention will be described by wayof a preferred embodiment thereof with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 shows a schematic block diagram illustrating handover andpacket scheduling according to a preferred embodiment of the presentinvention.

[0014]FIG. 2 shows a diagram of slow fading variation versus packettransmission time.

[0015]FIG. 3 shows a diagram of a safe packet allocation period.

[0016]FIG. 4 shows a diagram of a maximum allocation period foruncorrelated slow fading.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] The idea of the present invention is to determine a maximumpacket allocation period (i.e. transmission times) for at least onepossible base station, during which packet sending can be done withouthandover. In other words, during the maximum packet allocation period ofa cell, the probability is small that the cell becomes inappropriatewithin a transmission which takes place in the cell which was theappropriate one when the packet transmission started.

[0018] In order to improve packet scheduling, a Packet Scheduler (PS)should be able to separate or at least identify the Mobile Stations(MSs) that are in a handover area, i.e. near cell boarders. Thisinformation may be extracted by a Radio Network Controller (RNC) in aRadio Access Network (RAN) to which the MSs attach, on the basis of anactive Base Station (BS) update rate or from Energy per Chip toInterference Ratio (Ec/Io) values. The PS may be located in the RNC ormay be located in a Base Transceiver Station (BTS).

[0019] As shown in FIG. 1, an MS transmits measurement reporting towardsthe RAN. These measurements may be current SHO measurements. The RNCcollects and processes the measurements transmitted from the MS. On thebasis of these measurements, the RNC may identify whether the MS is inthe handover area. Moreover, from the processed measurements, the RNC isable to determine maximum allocation periods for possible BSs and todetermine an appropriate or even best BS out of the possible BSs on thebasis of the determined maximum allocation periods. During the maximumallocation period of the best cell the probability is small that anothercell becomes the best cell. The RNC communicates the information aboutthe maximum allocation periods and the appropriate or best BS to the PS.At first, the PS selects the appropriate or best BS for the MS, and thentransmits data packets for the MS to the selected BS with a bit-rate ofthe MS and an allocated period set in accordance with the maximumallocation period of the selected BS.

[0020] As can be understood from the foregoing, Hard HandOver (HHO) withadaptive allocation periods is used. The principle behind the idea ofsolely using HHO is due to the fact that packet transmission calls areshorter than circuit-switched calls. According to the above-describedalgorithm, during the transmission of a packet, a packet MS does notperform any handover update. At first, the BS is selected and then thePS allocates certain capacity.

[0021] In the following it will be described how the RNC determines themaximum allocation period for the MS.

[0022] An allocation period has to be set every time a new BS is chosen.An allocation period is also set when the previous allocation periodends. It may not be necessary to make handover after the allocationperiod has expired, but a new allocation period has to be estimated (ifthere is still data to transmit). This allocation period depends on theamount of change in the radio channel, which on its turn depends on theslow fading and the path loss change. The change due to path loss issmall compared to the change in shadow or slow fading, so the change inpath loss can be ignored.

[0023] For example, 3 and 50 km/h MS speeds are taken and the averagechange in the slow fading with respect to time is measured. From theslow fading formula, the correlation as a function of distance traveledby the MS is expressed as

ƒs(t)=ƒs(t−1)Rc+X(t){square root}{square root over (1−Rc ¹)}

[0024] where Rc is the correlation coefficient and X(t) is a normallydistributed random number. The random generator of X(t) takes intoaccount the wanted mean and standard deviation of the fading process. Rcis calculated as

Rc=exp(−dx/dcorr ln(1/ρ_(c)))

[0025] where dx is the distance interval between samples, dcorr is thede-correlation distance and ρ_(c) is the corresponding correlationcoefficient at dcorr. Thus, for 3 and 50 km/h, and 50 m dcorr andρ_(c)=0.5, the quantity sqrt(1−Rc²) is plotted in FIG. 2.

[0026] As it is illustrated in FIG. 2, as the packet transmission timeincreases, the chance of having larger shadowing increases. Hence, somemaximum transmission time is required to achieve some confidence thatthe appropriate or best BS does not change within the allocated period.

[0027] Since the handover area is quite large (i.e. 40 percent of usersor MSs are potentially candidates for handover in a macro cellenvironment), it is important that the handover is considered in thepacket scheduling. As identified above, the slow fading limits thetransmission period. In order to avoid interference in the case of“active set” (i.e. BS) changes, the slow fading should be considered inthe packet scheduling.

[0028] As a general rule, the maximum capacity allocation perioddetermined by the PS for a packet terminal or MS should be smaller thanthe average active BS update if HHO is used. This will ensure thatwithin the packet transmission period, there are no large changes in theslow fading.

[0029] In a typical WWW model, the average buffer size in one packetcall is roughly 12 kbytes. At 32 kbps, this requires an average of 3seconds of transmission. As the packet size has large variance, it isnot uncommon to have 10 seconds of transmission period. This can eitherbe segmented into smaller transmissions in different allocation periods,or be transmitted in a high bit rate bearer. Then the expectedtransmission period will be much less (i.e. <3 seconds). With respect toFIG. 2, at a speed of 50 km/h and a transmission time of 3 seconds witha slow fading with a standard deviation of 10 dB, an average of 2 dBchange is expected. This is quite acceptable.

[0030] The best way to obtain the maximum allocation period is bysetting some confidence such that within the defined period, there is alow probability of handover. This can be represented by FIG. 3. Asidentified earlier, the expected standard deviation as a function oftime for slow fading increases. Thus, the probability that the Ec/Iovalue of any BS exceeds that of the appropriate or best BS depends onthe MS speed, the initial Ec/Io difference (ΔEc/Io) between theappropriate or best BS and the other BSs, and the slow fading propertiesof the channel. By setting a confidence interval, the maximum allocationperiod can be determined.

[0031]FIG. 4 shows the safe period, i.e. the maximum allocation period,as a function of Ec/Io for various speeds and initial ΔEc/Io values withan uncorrelated shadow fading with a standard deviation of 10 dB.

[0032] The small maximum allocation periods do not have to be as smallas shown in FIG. 4, since the average difference in the case of anappropriate or best BS change is small if the maximum allocation periodis small.

[0033] In order to determine the maximum allocation period in the RNC,it is needed to generate a plot like that of FIG. 4 for everyenvironment. For the generation of such a plot, some knowledge about theproperties of the shadow fading (correlation length) is required. Thiscan be achieved by post-processing the measurements which are providedby the MS to the Radio Access Network (RAN). It must be possible thatthese measurements are available as time series, since only that way thecorrelation in time can be found. This fact may require some changes inthe data collection/filtering performed by the RNC. Since the shadowfading properties are depending on the environment, the measurements maybe collected on a cell basis, or the network processes the availabledata since the time of its deployment (auto-tuning feature). Accordingto the presently preferred embodiment, the current SHO measurements aresufficient to implement the algorithm described with reference to FIG.1.

[0034]FIG. 4 has to be taken into account in packet scheduling. For thispurpose, a simple addition in software is needed in the RNC.

[0035] While the invention has been described with reference to apreferred embodiment, the description is illustrative of the inventionand is not to be construed as limiting the invention. Variousmodifications and applications may occur to those skilled in the artwithout departing from the true spirit and scope of the invention asdefined by the appended claims.

1. A method of determining an allocation period for packet data in acommunication system, comprising the steps of: collecting measurementsfrom mobile stations; determining, on the basis of the collectedmeasurements, a maximum allocation period for at least one possible basestation for transmitting data packets to the mobile station; determininga base station out of the possible base stations on the basis of the atleast one determined maximum allocation period; and selecting thedetermined base station and a corresponding allocation period.
 2. Amethod according to claim 1, further comprising the step of detectingwhether a mobile station is in a handover area, wherein the determiningand selecting steps are performed if it is detected that the mobilestation is in a handover area.
 3. A method according to claim 2, whereinit is detected on the basis of an active BS update rate whether themobile station is in a handover area.
 4. A method according to claim 2,wherein it is detected whether the mobile station is in a handover area,on the basis of Energy per Chip to Interference ratio values containedin the collected measurements.
 5. A method according to claim 1, whereinthe collected measurements comprise SHO measurements conducted by themobile stations.
 6. A method according to claim 1, wherein out of thecollected measurements slow fading properties of a channel are derived.7. A method according to claim 6, wherein out of the collectedmeasurements differences between the Energy per Chip to Interferenceratio value of a possible base station in question to that of otherpossible base stations are obtained.
 8. A method according to claim 7,wherein the at least one maximum allocation period is determined fromthe slow fading properties and the differences between the Energy perChip to Interference ratio values.
 9. A method according to claim 7,wherein the speed of the mobile station is determined out of thecollected measurements.
 10. A method according to claim 9, wherein theat least one maximum allocation period is determined from the slowfading properties, the differences between the Energy per Chip toInterference ratio values and the speed of the mobile station.
 11. Amethod according to claim 1, wherein the measurements are collected andprocessed on a cell basis.
 12. A method according to claim 1, whereinthe measurements are collected and processed over a long period of time.13. A method according to claim 1, wherein the corresponding allocationperiod does not exceed the maximum allocation period of the selectedbase station.
 14. A method according to claim 1, wherein the best basestation is selected if the longest maximum allocation period isrequired.
 15. A control device for determining an allocation period forpacket data in a communication system, said device comprising: a controlentity (RNC) for collecting measurements from mobile stations, fordetermining, on the basis of the collected measurements, a maximumallocation period for at least one possible base station fortransmitting data packets to the mobile station, and for determining abase station out of the possible base stations on the basis of the atleast one determined maximum allocation period; and a scheduling entity(PS) for selecting the determined base station and a correspondingallocation period.
 16. A control device according to claim 15, whereinthe control entity is further adapted to detect whether a mobile stationis in a handover area, and, if said control entity detects that themobile station is in a handover area, it performs the determination ofthe at least one maximum allocation period.
 17. A control deviceaccording to claim 16, wherein said control entity is arranged to detecton the basis of an active BS update rate whether the mobile station isin a handover area.
 18. A control device according to claim 16, whereinsaid control entity is arranged to detect whether the mobile station isin a handover area, on the basis of Energy per Chip to Interferenceratio values contained in the collected measurements.
 19. A controldevice according to claim 15, wherein the collected measurementscomprise SHO measurements conducted by the mobile stations.
 20. Acontrol device according to claim 15, wherein said control entity isarranged to derive slow fading properties of a channel out of thecollected measurements.
 21. A control device according to claim 20,wherein said control entity is arranged to obtain, out of the collectedmeasurements, differences between the Energy per Chip to Interferenceratio value of a possible base station in question to that of otherpossible base stations.
 22. A control device according to claim 21,wherein said control entity is arranged to determine the at least onemaximum allocation period from the slow fading properties and thedifferences between the Energy per Chip to Interference ratio values.23. A control device according to claim 21, wherein said control entityis arranged to determine the speed of the mobile station out of thecollected measurements.
 24. A control device according to claim 23,wherein said control entity is arranged to determine the at least onemaximum allocation period from the slow fading properties, thedifferences between the Energy per Chip to Interference ratio values andthe speed of the mobile station.
 25. A control device according to claim15, wherein said control entity collects and processes the measurementson a cell basis.
 26. A control device according to claim 15, whereinsaid control entity collects and processes the measurements over a longperiod of time.
 27. A control device according to claim 15, wherein saidscheduling entity selects the corresponding allocation period such thatit does not exceed the maximum allocation period of the selected basestation.
 28. A control device according to claim 15, wherein the bestbase station is selected if the longest maximum allocation period isrequired.
 29. A control device according to claim 15, wherein saidcontrol entity and said scheduling entity are located in the samephysical network element.
 30. A control device according to claim 15,wherein said control entity and said scheduling entity are located indifferent physical network elements and said scheduling entity isarranged to receive information about the determined base station andassociated maximum allocation period from the control entity.
 31. Acommunication system for determining an allocation period for packetdata, said system comprising: a control entity (RNC) for collectingmeasurements from mobile stations, for determining, on the basis of thecollected measurements, a maximum allocation period for at least onepossible base station for transmitting data packets to the mobilestation, and for determining a base station out of the possible basestations on the basis of the at least one maximum allocation period; anda scheduling entity (PS) for selecting the determined base station and acorresponding allocation period.
 32. A system according to claim 31,wherein the control entity is further adapted to detect whether a mobilestation is in a handover area, and, if said control entity detects thatthe mobile station is in a handover area, it performs the determinationof the at least one maximum allocation period.
 33. A system according toclaim 32, wherein said control entity is arranged to detect on the basisof an active BS update rate whether the mobile station is in a handoverarea.
 34. A system according to claim 32, wherein said control entity isarranged to detect whether the mobile station is in a handover area, onthe basis of Energy per Chip to Interference ratio values contained inthe collected measurements.
 35. A system according to claim 31, whereinthe collected measurements comprise SHO measurements conducted by themobile stations.
 36. A system according to claim 31, wherein saidcontrol entity is arranged to derive slow fading properties of a channelout of the collected measurements.
 37. A system according to claim 36,wherein said control entity is arranged to obtain, out of the collectedmeasurements, differences between the Energy per Chip to Interferenceratio value of a possible base station to that of other possible basestations.
 38. A system according to claim 37, wherein said controlentity is arranged to determine the at least one maximum allocationperiod from the slow fading properties and the differences between theEnergy per Chip to Interference ratio values.
 39. A system according toclaim 37, wherein said control entity is arranged to determine the speedof the mobile station out of the collected measurements.
 40. A systemaccording to claim 39, wherein said control entity is arranged todetermine the at least one maximum allocation period from the slowfading properties, the differences between the Energy per Chip toInterference ratio values and the speed of the mobile station.
 41. Asystem according to claim 31, wherein said control entity collects andprocesses the measurements on a cell basis.
 42. A system according toclaim 31, wherein said control entity collects and processes themeasurements over a long period of time.
 43. A system according to claim31, wherein said scheduling entity selects the corresponding allocationperiod such that it does not exceed the maximum allocation period of theselected base station.
 44. A system according to claim 31, wherein thebest base station is selected if the longest maximum allocation periodis required.
 45. A system according to claim 31, wherein said controlentity and said scheduling entity are located in the same physicalnetwork element.
 46. A system according to claim 31, wherein saidcontrol entity and said scheduling entity are located in differentphysical network elements and said scheduling entity is arranged toreceive information about the determined base station and associatedmaximum allocation period from the control entity.
 47. A communicationsystem according to claim 31, wherein the control device is located in aradio access network to which the mobile stations attach.
 48. Acommunication system according to claim 47, wherein the control entitycollects and processes the measurements since the time of the deploymentof the radio access network.